1. Field of the Invention
The present invention relates in general to substrate polishing apparatuses, and relates in particular to a polishing apparatus suitable for use in polishing a substrate which requires a high degree of cleanliness, such as a semiconductor wafer, a glass substrate and a liquid crystal panel.
2. Description of the Related Art
In recent years, as semiconductor devices become highly integrated, circuit lines have become microsized, and interline spacing has also been greatly reduced. To produce finely resolved lines by photolithographic techniques, a higher degree of flatness is required in the substrate because of the shallow depth of focus of the optical system inherent in stepper image reproduction system. Such flatness requirements are the same for preparing glass substrates for masking or for liquid crystal panels. A method of obtaining a flat surface on a substrate such as semiconductor material is chemical mechanical polishing (CMP) in which a substrate held on a substrate holding device is polished by pressing the substrate against a polishing cloth mounted on a rotating turntable and supplying a polishing solution containing abrasive particles on the polishing cloth.
FIG. 11 is a schematic overall plan view of a conventional polishing facility. The dual-line facility is comprised of substrate storage sections 200a, 200b for storing substrates to be polished; at least two polishing units comprised of a first polishing unit 206a, and a second polishing unit 206b, each having its own turntable 202 and own top ring 204; cleaning devices 208a.about.208d for washing and drying the polished substrates which have been processed in the polishing units; a transport device 210 for transporting the cleaned substrates between processing stations; and inverting devices 212a, 212b for inverting the substrate.
In each line of the polishing facility, a substrate transported from the storage sections 200a, 200b by the transport device 210 is inverted by the inverting devices 212a, 212b, and the inverted substrate is transported by the transport device 210 to the polishing units 206a, 206b for polishing the bottom surface of the substrate. The polished substrate is transported back to the inverting devices 212a, 212b to again invert the substrate to orient the polished surface upwards, and the inverted substrates are transported back to the cleaning devices 208a.about.208d for washing and drying.
As shown in FIG. 12, each polishing unit 206a or 206b is comprised of, for example: a drive device 233 for driving a turntable 202 having a polishing cloth (polishing tool) 230 adhered thereon; a top ring device 204 having a top ring head 234 for vacuum chucking of a substrate for pressing the substrate against the polishing cloth 230 with a specific pressure; a dressing device 240 having a dressing head 238 for conditioning the polishing cloth 230; and a polishing solution supply device 242 for supplying a polishing solution containing water and abrasive grains to turntable 202 and top ring head 234.
Top ring head 234 is connected to the bottom end of a main shaft 246 supported through an end of a swing arm 244 in such a way as to permit vertical and rotational movements of the main shaft 246. Polishing is performed by the top ring head 234 by first receiving a substrate from a substrate delivery device (not shown) disposed on a lateral-side of the turntable 202, and then holding and rotating the substrate by means of the rotating main shaft 246. In the meantime, the dressing head 238 is also similarly supported through the end of the swing arm 244 at the bottom end of the main shaft 246 so as to be rotatable and movable vertically to perform dressing of the polishing cloth 230.
As can be seen in FIG. 12, the polishing units 206a, 206b perform polishing by the action of the independently-rotatable turntable 202. Therefore, at the center of the turntable 202, there is no displacement of the cloth and no polishing can be performed. So, polishing is performed off-center on the turntable 202. In such an arrangement, the diameter of the turntable 202 becomes more than twice the substrate diameter.
The drive mechanism for the main shafts 246 for rotating the top ring device 204 and the dressing device 240 is comprised of a motor 250 at the proximal end of the swing arm 244, while at its free end, it is comprised of a drive pulley 254 rotated by the gear inside a gear box 252; a follower pulley 256 rotating with the main shaft 246; and a timing belt 258 to transmit the drive force between the drive pulley 254 and the follower pulley 256.
An encoder 260 is provided at the top end of the motor shaft of the motor 250, thereby receiving feedback signals to optimally control the rotation of the top ring head 234 or the dressing head 238 at a predetermined cycle by way of the main shaft 246.
One of the operational problems in such a conventional polishing apparatus is that the top ring head is supported by the swing arm disposed on the lateral-side of the turntable, and substrate handling is performed by swinging the swing arm. Therefore, it is necessary that no mechanical interference be encountered within the swinging radius of the swing arm. Furthermore, because the facility also includes a dressing device, mechanical interference must be avoided for both devices, and the result is that there is a need to allocate a fairly large installation space.
Also, the top ring head is supported at one end of the swing arm in a cantilever manner, and because of the weight of the additional devices for generating rotating and pressing actions, the swing arm has to be constructed with a high degree of stiffness. However, if the strength of the swing arm is achieved by increasing its stiffness, the swing arm itself becomes massive, increasing weight so that this approach has its limit. The same problem are encountered for the dressing head.
A second operational problems of the conventional polishing apparatus is that because the diameter of the turntable must be more than twice the diameter of the substrate, a large floor space is necessary, and the overall polishing apparatus also becomes very large, resulting in a high attendant capital cost. This problem would become worse with the modern tendency to increase the diameter of the substrate.
Therefore, there have been proposals for replacing or supplementing such a polishing apparatus with a polishing unit of a smaller diameter to move in a circular translation pattern. In this case, the total polishing surface on the polishing cloth is subjected to an identical movement to be utilized effectively in removing the substrate material so that the size of the polishing tool needs to be about the same as the substrate size. However, the action of the driver to produce such a circular translation motion requires eccentric positioning of the drive shaft with respect to the center of the polishing table, and in such an arrangement, if the drive shaft is not properly balanced, the entire apparatus becomes vulnerable to vibration. Additionally, because the center of gravity of the drive shaft and that of the polishing table are not coincident, table rotation produces dynamic instability and results in vibrations. The overall effect is that the polishing operation becomes unstable and environmental problems will be generated.
A third problem in the conventional polishing apparatus is that, because the motor rotation is transmitted to the main drive shafts (for top ring head 234, dressing head 238) through the timing belts, feedback response is slow in providing a real time control of the shafts' rotations. For example, response delays can occur in detecting the resistive forces being experienced by the top ring head 234 and dressing head 238 during the polishing or dressing operation, or conversely, delays can occur in transmitting the response change in drive force to the main shaft. Further, when vibrations are generated in any of the swing arms, gear boxes, and transmission systems, including the timing belts, such vibrations are transmitted to the polishing tools (top ring head and dressing head), and consequently, fluctuations in rotation of the polishing tools introduce inconsistencies in performance of the polishing tool, thereby interfering with the process of creating uniform flatness.
A fourth problem in the conventional polishing apparatus is that, since the downward-facing surface of the substrate is polished in the polishing process, which is followed by a cleaning process using a cleaning device arranged to clean the polished surface of the substrate, it is necessary to interpose a substrate-inverting step between the two processes. This arrangement not only contributes to increasing the production cost, but the inverting process is time-consuming as well as inefficient.